[SPARK-19537] Move pendingPartitions to ShuffleMapStage.

The pendingPartitions instance variable should be moved to ShuffleMapStage,
because it is only used by ShuffleMapStages. This change is purely refactoring
and does not change functionality.

I fixed this in an attempt to clarify some of the discussion around #16620, which I was having trouble reasoning about.  I stole the helpful comment Imran wrote for pendingPartitions and used it here.

cc squito markhamstra jinxing64

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #16876 from kayousterhout/SPARK-19537.